Optical imaging systems are commonly used to decipher data symbols printed on objects in order to identify the objects or to obtain information relating to the object. A bar code symbol is a common one-dimensional form of symbology, and typically comprises a pattern of vertical bars of various widths separated by spaces of various widths. Because the bar and space elements have different light reflecting characteristics, a reader can convert a symbol into an electrical signal by analyzing the light reflected from the symbol. The electrical signal can then be analyzed and decoded to provide an alphanumeric representation of the symbol, which can contain certain information about the object. Bar code symbols of this type are now in common usage in various applications, such as inventory control, point of sale identification, or logistical tracking systems.
Because conventional one-dimensional symbology requires a relatively large amount of space to convey a correspondingly small amount of data, so-called two-dimensional bar code symbologies have been developed. A two-dimensional symbology may comprise a matrix that occupies a uniform amount of space having a generally rectangular or square shape. Instead of bars and spaces, round or square marks disposed at particular rows and columns of the matrix correspond to the information being conveyed. As a result, a two-dimensional matrix symbology can include significantly more data within a given volume of space than a conventional one-dimensional bar code.
Many bar code symbols are printed on labels, which are affixed to objects, boxes, etc. Alternatively, bar codes can be set in relief or formed within the surface of an object. This can be done by stamping, engraving, etching, milling, molding, or by other known methods. The symbols can be either raised from the surface or depressed within the surface. Such relief formed symbols can be more durable, less expensive, and provide other benefits of typical bar code labels. However, such relief formed symbols can be difficult to read using currently available non-contact scanning techniques, because the contrast between raised and recessed portions of the symbol is generally low. For example, if a laser scanner scans a relief formed symbol, both the high and low regions of the symbol reflect the scanning beam substantially equally thereby making differentiation between the high and low regions very difficult.
One particular application of a two-dimensional symbology utilizes a two-dimensional symbol in the manufacture of microelectronics devices such as certain flat panel displays. Typically, a symbol is formed directly on a display substrate, which is usually glass, by printing with appropriate ink or other suitable technique. Alternatively, the symbol could be formed directly onto the substrate by laser etching or other suitable precision relief forming process. Because a two-dimensional symbology can compress a large amount of data within a relatively small dimensional space, the symbol can store a unique identifier code for the substrate, including such information as serial number, lot number, batch number, model number, and/or customer code. As such, the symbols can be used to automate the manufacturing or testing processes, and may also enable manufacturers to protect against component theft or forgery.
Generally, such symbols formed on typical substrates may be readable with conventionally known equipment. However, an early processing step in the manufacture of certain flat panel displays, such as liquid crystal displays, necessarily covers the entire substrate, including the symbol, with a thin-film coating of a highly reflective metal such as chromium. Because such metal coatings are usually conformal in nature and very thin, the relief aspects of the symbol formed on the substrate are usually preserved in the thin-film coating thereby forming a buried relief symbol. A significant drawback of such buried relief symbols is that they generally have little or no light reflecting contrast and, as a result, can be very difficult to image. Because the symbol characters are buried beneath a metal thin-film, any color difference that existed between the characters and the substrate can no longer be utilized for imaging the symbol. To compound this problem, the metal thin-film usually has a shiny surface finish that tends to further obscure the symbol characters.